a. Field of the Invention
This invention relates to a light beam scanning device.
B. Description of the Prior Art
A light beam scanning device in which a light beam is caused to impinge on a deflecting mirror such as rotatable polygonal mirror or galvano-mirror to thereby provide a deflected beam has been used in such an apparatus as image original reader apparatus or laser beam printer apparatus. In the light beam scanning device used in such apparatus, it is desirable that the scanning line depicted on a scanned surface by the deflected beam be coincident with a predetermined scanning line on the scanned surface. More particularly, the light beam scanning device is desired in which the deflected beam will not move on a line different from the predetermined line on the scanned surface. Such a phenomenon of the deflected beam which moves out of alignment with the predetermined line on the scanned surface occurs in such cases where, for example, the rotatable polygonal mirror is not disposed with the deflecting mirror surfaces thereof parallel to the rotary shaft thereof or where the rotary shaft is not mounted with sufficient accuracy. Therefore, in order to prevent occurrence of such phenomenon, the rotatable polygonal mirror or the galvano-mirror should be manufactured with extreme accuracy, but with the cost or other problem in view, it is difficult to manufacture the rotatable polygonal mirror or the galvano-mirror with sufficient accuracy and if such mirror could at all be manufactured with sufficient accuracy, it would be very difficult to maintain the accuracy for a long time.
For this reason, various scanning devices have heretofore been proposed which have succeeded in preventing or reducing the occurrence of the above-noted phenomenon.
For example, U.S. Pat. No. 3,750,189 discloses a scanning optical system which prevents occurrence of such phenomenon by skilful use of a cylindrical lens. Also, our U.S. Application Ser. No. 569,608 and German Pat. application P 25 17 821.6 disclose a scanning optical system including an anamorphic optical system disposed in the path of the deflected beam, the anamorphic optical system being effective only in a direction perpendicular to the direction of deflection of the deflected beam, thereby reducing occurrence of the aforementioned phenomenon.
The present invention relates to an improvement in the scanning optical system covered by our prior applications.
The improvement lies in that occurrence of the nonlinear scanning line which would otherwise result from the use of a prism anamorphic optical system as the anamorphic optical system, namely, distortion or deflection of the scanning line, may be prevented or reduced. The above-noted phenomenon of the deflected beam which moves on a line different from a predetermined line (hereinafter, this phenomenon will be referred to as "tilt" and the prevention or reduction of such phenomenon will be called "correction of the tilt") may be reduced in its possibility of occurrence by the use of a prism anamorphic optical system, but due to the optical characteristic of the prism forming the prism anamorphic optical system, the deflected beam is displaced in a direction perpendicular to the direction of deflection by different amounts, depending on the position whereat the angle of deflection lies, so that distortion or deflection occurs to the scanning line.